This invention relates to a covering for heat treatment of tissue and, in particular, covers having a substantial portion of the cover in non-contact with the tissue and capable of delivering heat to the tissue. The covering preferably controls the temperature, humidity and other aspects of the environment at the tissue.
Wounds in general, as used in this context, are breaks in the integrity of the skin of a patient. Wounds may occur by several different mechanisms. One such mechanism is through mechanical traumatic means such as cuts, tears, and abrasions. There are many instruments of casualty for mechanical wounds, including a kitchen bread knife, broken glass, gravel on the street, or a surgeon""s scalpel. A different mechanism cause for mechanical wounds is the variable combination of heat and pressure, when the heat alone is insufficient to cause an outright burn. Such wounds that result are collectively referred to as pressure sores, decubitus ulcers, or bed sores, and reflect a mechanical injury that is more chronic in nature.
Another type of mechanism causing a wound is vascular in origin, either arterial or venous. The blood flow through the affected region is altered sufficiently to cause secondary weakening of the tissues which eventually disrupt, forming a wound. In the case of arterial causes, the primary difficulty is getting oxygenated blood to the affected area. For venous causes, the primary difficulty is fluid congestion to the affected area which backs up, decreasing the flow of oxygenated blood. Because these wounds represent the skin manifestation of other underlying chronic disease processes, for example, atherosclerotic vascular disease, congestive heart failure, and diabetes, these vascular injuries also are chronic in nature, forming wounds with ulcerated bases.
Traditional wound coverings, such as bandages, are used to mechanically cover and assist in closing wounds. Such bandages typically cover the wound in direct contact with the wound. This may be acceptable for acute, non-infected traumatic wounds, but it must be kept in mind that direct bandage contact with a wound can interfere with the healing process. This interference is particularly prevalent for chronic ulcerated wounds because of the repeated mechanical impact and interaction of the bandage with the fragile, pressure sensitive tissues within the wound.
The benefits of application of heat to a wound are known, and documented benefits include: increased cutaneous and subcutaneous blood flow; increased oxygen partial pressure at the wound site; and increased immune system functions, both humoral and cell mediated, including increased migration of white blood cells and fibroblasts to the site.
However, heat therapy for the treatment of wounds, either infected or clean, has been difficult to achieve in practice. For instance, heating lamps have been used, but these results in drying of wounds, and in some cases, even burning tissue from the high heat. Due to these and other difficulties, and since most acute wounds usually heal over time, physicians no longer consider the application of heat to the wound as part of the treatment process. The thinking among medical personnel is that any interference in a natural process should be minimized until it is probable that the natural process is going to fail. Additionally, the availability of antibiotics for use in association with infected wounds has taken precedence over other therapies for the treatment of chronic wounds and topical infections.
In French patent number 1,527,887 issued Apr. 29, 1968 to Veilhan there is disclosed a covering with rigid oval dome, its edge resting directly on the patient""s skin. One aspect of the Veilhan wound protector is a single oval heating element resting on the outer surface of the rigid dome, positioned at the periphery of the rigid dome. Veilhan does not discuss the heating aspect other than to state that it is a component.
The benefits of controlling other environmental parameters around the wound site are not as well known. Controlling the humidity at the wound site and the benefits of isolating the wound have not been extensively studied and documented.
While the benefit of applying heat to wounds is generally known, the manner of how the heat should be used or applied is not known. Historically, heat was applied at higher temperatures with the goal of making the wound hyperthermic. These higher temperatures often resulted in increasing tissue damage rather than their intended purpose of wound therapy and healing. There is a need for appropriate wound care management incorporating a heating regimen that is conducive to wound healing, yet safe and cost effective.
The present invention disclosed herein approaches the treatment of wounds with heat based on an understanding of physiology. The normal core temperature of the human body, what will be defined herein for purposes of this disclosure, is 37xc2x0 C.xc2x11xc2x0 C. (36xc2x0-38xc2x0C.), which represents the normal range of core temperatures for the human population. For purposes of discussion and this disclosure, normal core temperature is the same as normothermia. Depending on the environmental ambient temperature, insulative clothing and location on the body, skin temperature typically ranges between about 32xc2x0 C. and about 37xc2x0 C. From a physiologic point of view, a 32xc2x0 C. skin temperature of the healthy distal leg is moderate hypothermia. The skin of the distal leg of a patient with vascular insufficiency may be as low as 25xc2x0C. under normal conditions, which is severe hypothermia.
A fundamental physiologic premise is that all cellular physiologic functions, biochemical and enzymatic reactions in the human body are optimal at normal body core temperature. The importance of this premise is seen in how tightly core temperature is regulated. Normal thermoregulatory responses occur when the core temperature changes as little as xc2x10.1xc2x0 C. However, the skin, as noted above, is usually hypothermic to varying degrees. For example, the skin of the torso is usually only slightly hypothermic, whereas the skin of the lower legs is always hypothermic. Consequently, wounds and ulcers of the skin, regardless of location, are usually hypothermic. This skin hypothermia slows cellular functions and biochemical reactions, inhibiting wound healing.
The effects of hypothermia on healing are well known. A number of regulatory systems within a human are affected, such as the immune system and coagulation, with both platelet function as well as the clotting cascade affected. Patients with hypothermic wounds experience more infections which are more difficult to treat, have increased bleeding times and have been shown to require more transfusions of blood. All of these complications increase morbidity and the cost of patient care and, to a lesser extent, increase the likelihood of mortality.
One purpose of the present invention is to raise the wound tissue and/or periwound tissue temperatures toward normothermia to promote a more optimal healing environment. The present invention is not a xe2x80x9cheating therapyxe2x80x9d, per se, where it is the intent of xe2x80x9cheating therapyxe2x80x9d to heat the tissue above normothermia to hyperthermia levels. Rather, the present invention is intended to bring the wound and periwound tissues toward normothermia without exceeding normothermia.
The medical community has not historically considered normothermic heating to be therapeutic. Many physicians feel that hypothermia is protective and, therefore, desirable. Studies with the present invention would indicate that this widely held belief that hypothermia is at least benign or possibly beneficial is incorrect with regard to wound healing.
The present invention is a wound covering for application to a selected treatment area of a patient""s body that includes, at least as a portion of the selected treatment area, a selected wound area. The selected treatment area may also include a portion of the area immediately proximate to the wound area referred to as the periwound area. The wound covering comprises a heater suitable for providing heat to the selected treatment area, an attachment for attaching the heater in a non-contact position over the selected treatment area, and a heater controller, connected to the heater and including a power source for the heater, for controlling the temperature of the heater in a temperature range from above ambient temperature to about 38xc2x0 C. Ambient temperature is that temperature of the environment immediately around the selected treatment area not a portion of the patient""s body, i.e., the bed, the air in the room, the patient""s clothing, etc.
The heater is selectable from among several types of heat sources such as warmed gases directed over the selected treatment area and electrical heater arrays placed proximate the selected treatment area. Electrical heater arrays are adaptable for construction into a layer of variable proportion and geometry or as a point source. The present invention anticipates the ability to provide several different sizes and geometric configurations for the heater. The present invention is flexible in being able to provide uniform heating over the entire selected treatment area or provide a non-uniform heating distribution over selected portions of the selected treatment area. Alternate heat source embodiments could include warm water pads, exothermic chemical heating pads, phase-change salt pads, or other heat source materials.
The present invention anticipates that the controller is able to control both the temperature and the duration of the application of heat. This control may extend from manual to fully automatic. Manual control anticipates the controller maintaining the heater temperature at an operator-selected temperature for as long as the operator leaves the heater on. More automatic modes provide the operator an ability to enter duty cycles, to set operating temperatures, as well as to define therapy cycles and therapeutic sequences. As used herein, a duty cycle is a single on cycle when heating of the heater is occurring, measured from the beginning of the on cycle to the end of that on cycle. A heater cycle is a single complete on/off cycle measured from the beginning of a duty cycle to the beginning of the next duty cycle. Consequently, a duty cycle may also be represented in a percentage of, or as a ratio of the time on over the time off. A plurality of heater cycles are used to maintain heater temperature around a selectable temperature set point during a therapy cycle which is defined as an xe2x80x9conxe2x80x9d period, composed of a plurality of heater cycles, and an xe2x80x9coffxe2x80x9d period equivalent to the heater remaining off for an extended period of time. A therapeutic sequence, as used herein, is a longer period of time usually involving a plurality of therapy cycles spread out over an extended period of time, the most obvious being a day in length. The present invention anticipates the use of any period of time as a therapeutic sequence and involving one, or more than one therapy cycles.
The present invention also anticipates programmability for a number of modalities including peak heater temperature for a duty cycle and/or therapy cycle, average heater temperature for a duty cycle and/or therapy cycle, minimum heater temperature for a heater cycle and/or therapy cycle, ratio of duty cycle, length of therapy cycle, number of duty cycles within a therapy cycle, and number of therapy cycles in a therapeutic sequence. Different duty cycles within a therapy cycle may be programmed to have different peak heater temperatures and/or heater cycles may have average heater temperatures over that therapy cycle. Different therapy cycles within a therapeutic sequence may be programmed to have different peak heater temperatures and/or average heater temperatures over each therapy cycle. The wound covering control is operator-programmable or may have preprogrammed duty cycles, therapy cycles, and therapeutic sequences selectable by the operator.
A preferred form of the wound covering includes an attachment as a peripheral sealing ring which, in use, completely surrounds the area of the wound and periwound, i.e., the selected treatment area. The upper surface of the peripheral sealing ring is spanned by a continuous layer which is preferably transparent and substantially impermeable, although the present invention also anticipates the use of a gas permeable layer suitable for some applications. Once in position, the sealing ring and the layer define a wound treatment volume which surrounds the wound. Additionally, the layer spanning the peripheral sealing ring may be sealed about the periphery of the sealing ring and act as a barrier layer over the wound treatment volume. Optionally, the heater may be incorporated into the barrier layer or the barrier layer may be incorporated into the heater. An adhesive and a suitable release liner is applied to the lower surface of the peripheral sealing ring to facilitate the application of the wound covering to the patient""s skin.
The barrier layer may include a pocket adapted to receive an active heater. An alternate form of the invention provides for the transport of heated air from a remote heater source to the wound treatment volume. In the active heater embodiments a thermostat and/or a pressure-activated switch may be used to control the heating effects of the heater. Passively heated embodiments are contemplated as well. These passive versions of the device include the use of thermally insulating coverings which retain body heat within the treatment volume. These reflectors or insulators may be placed in a pocket formed in the barrier layer. Each of these heated embodiments promote wound healing by maintaining the wound site at a generally elevated, but controlled, temperature.
In general, the peripheral sealing ring is made from an absorbent material which may act as a reservoir to retain and/or dispense moisture into the treatment volume increasing the humidity at the wound site. The reservoir may also contain and deliver medicaments and the like to promote healing.
The present invention is designed to elevate the temperature of the hypothermic skin and subcutaneous tissue of the selected treatment area to a temperature which is close to normothermia. The purpose of this device is to create within the wound and periwound tissues of the selected treatment area a more normal physiologic condition, specifically a more normothermic condition, which is conducive to better wound healing. The present invention anticipates the use of an active heater, but the role of the heater can better described as xe2x80x9cguardingxe2x80x9d against heat loss by providing a heat source to counteract the effects of heat loss.
The concept of a guard heater is straightforward. The guard heater is heated to approximately the same temperature as the adjacent heated body. Since heat must flow down a temperature gradient, it can only be lost to a cooler surface. The guard heater is not cooler than the adjacent body and, therefore, cannot accept heat from the adjacent body. The normal temperature gradient for tissue goes from about 37xc2x0 C. deep in the body""s core down to about 32xc2x0 C. at the skin""s surface. With a guard heater in place, heat loss directly from the wound and periwound tissue surfaces is markedly diminished. This decrease in local heat loss provides for the zone of 37xc2x0 C. core temperature to move outward toward the skin, narrowing the gradient from core temperature to surface temperature as the zone of core temperature approaches the surface in the area of the guard heater. The guard heater behaves very much like a perfect insulator, providing a circumstance suitable for warming of the wound with heat flowing from the core. The guard heater of the present invention has an additional advantage over neat-perfect passive insulation in that near-perfect insulation would require the use of several inches of bulky insulating material. Such bulkiness in a wound dressing is not practical for proper wound care.
The xe2x80x9cguardxe2x80x9d heater of the present invention, for example in use on wounds below the knee where vascular insufficiency may occur, operates from the above ambient temperature to 38xc2x0 C., which includes the range of xe2x80x9cnormothermiaxe2x80x9d. Obviously, because of the thermal mass of the leg, the blood flow through the leg and inherent inefficiencies of heat transfer, the resulting wound and periwound tissue temperatures usually remain less than the operating temperature of the xe2x80x9cguardxe2x80x9d heater, and probably less than the patient""s core temperature.
In contrast, typical local heating therapy (e.g. hot water bottles, hot water pads, chemical warmers, infrared lamps) deliver temperatures greater than 46xc2x0 C. to the skin. The goal of traditional heating therapy is to heat the tissue above normal, to hyperthermic temperatures.
The present invention differs from infrared lamps in two ways. First, the present invention includes a dome over the wound that is relatively impermeable to water vapor transmission. After application of the bandage, moisture from the intact skin or wound evaporates, and air within the dome quickly reaches 100% relative humidity. The interior of the present invention is now warm and humid. For example, a 2.5 square inch bandage at 28xc2x0C. requires only 0.0014 g of water to reach saturation. When the air is thus saturated, no further evaporation can occur and, therefore, no drying of the wound can occur. This equilibrium will be maintained as long as the bandage is attached to the patient.
When heat is provided by the preferred embodiment of the present invention, the absolute amount of water needed to reach 100% relative humidity is slightly increased since warm air has a greater capacity for holding moisture. However, the air within the dome of the bandage still reaches water vapor saturation very quickly, and no further evaporation occurs. For example, a 2.5 square inch bandage of the present invention at 38xc2x0 C. requires only 0.0024 g of water to reach saturation. Excess moisture is absorbed by the foam ring, but still is retained within the bandage. The enclosed dome design maintains 100% humidity over the wound which also prevents evaporation due to the heat. As long as the humidity is retained within the bandage, heating therapy could theoretically be continued indefinitely without causing the wound to dry. In contrast, when using infrared lamps, the wounds are open and exposed to the environment. The result is excessive drying of the wound, increasing tissue damage.
Secondly, the present invention operates at low temperatures, from above ambient to about 38xc2x0 C. This causes only minimal heating of the skin. In contrast, infrared lamps operate at temperatures in excess of 200xc2x0 C. These lamps heat the wound to hyperthermic temperatures which can cause thermal damage to the tissue of the wound.
At the low (normothermic) operating temperatures of the present invention, the heat transfer to the skin is minimal. The low wattage heater, the inefficiencies of the heat transfer into the tissue, the thermal mass of the tissue and the blood flow (even if markedly reduced), all prevent the wound temperature from reaching the heater temperature. Hypothermic wound tissue is warmed as a result of xe2x80x9cmigrationxe2x80x9d of the body""s core temperature zone toward the local wound area.
The following data document the tissue temperature resulting from a 38xc2x0 C. heater of the present invention on:
When warmed with a 38xc2x0 C. heater, wounds on poorly perfused legs reach stable average temperatures of 32-33xc2x0 C. In contrast, normally perfused skin reaches 36xc2x0 C. It is important to note that these data are contradictory to the assumption that poorly perfused tissue would reach a higher temperature than normally perfused tissue. This result substantiates the physiologic finding that the xe2x80x9cmigrationxe2x80x9d of the core temperature zone toward the local wound zone, decreasing the gradient difference between the core and surface temperatures, is the cause for the observed increased wound temperatures. Core temperature regulation is heavily dependent on perfusion, and migration of the core temperature zone is also heavily dependent on perfusion. At no point in time did the poorly perfused tissue reach normothermia. Poorly perfused legs are colder than normally perfused legs, and, thus, poorly perfused legs constitute a substantially deeper heat-sink.
A wound-healing pilot study is under way, studying patients with chronic arterial and/or venous ulcers of the lower leg. These patients have suffered from these ulcers for many months and, in some cases, even years, despite aggressive medical and surgical therapy. Of 29 patients enrolled, 24 have completed the study protocol or are still being treated. Of these 24 patient""s, 29% are completely healed, and 38% show a significant reduction of the wound size with 2-5 weeks of receiving therapy with the present invention.
A known consequence of restoring normothermia to tissues is to induce some degree of vasodilatation which increases local blood flow. Preliminary data collected during trials of the present invention, studying the effects of the present invention on normal subjects and on wound healing, has borne this out. An added effect has been to increase the partial pressure of oxygen in the subcutaneous tissues (PsqO2), which is an indirect indicator of the status of the tissue. The higher the PsqO2, the greater the likelihood the tissue will benefit and improve the healing process. The results of some of these studies are presented in Tables 1-4.
In conducting the studies presented in Tables 1-4, a wound covering according to the present invention is placed over the skin. The temperature of the subcutaneous tissue is then measured over time. From xe2x88x9260 minutes to the 0 minute mark, the heater is off in order to obtain a baseline temperature. At the 0 minute mark the heater is activated and its temperature kept constant over the next 120 minutes when it is turned off. Temperature measurements were taken during this 120 minute period and for an additional 180 minutes after turning the heater off. As shown in Table 1, with activation of the heater to 38xc2x0 C., the subcutaneous tissue temperature rapidly rose from about 34.3xc2x0 C. to about 36xc2x0 C. over the first 30 minutes. The temperature of the subcutaneous tissue continued to slowly raise over the next 90 minutes to a temperature of about 36.7xc2x0 C. After turning the heater off, the temperature of the subcutaneous tissue fell to about 35.9xc2x0 C. and held this temperature fairly uniformly for at least the next 120 minutes.
Table 2 presents the skin temperature data collected from within the wound cover of the present invention for the same periods as those in Table 1. The general curve shape is similar to the subcutaneous tissue temperature curve. The baseline temperature at the 0 minute mark was about 33.5xc2x0 C. After turning the heater on to 38xc2x0 C., the skin temperature rose rapidly to about 35.8xc2x0 C. in the first 30 minutes, then slowly rose to about 36.2xc2x0 C. by the end of the 120 minute heating period. After turning the heater off, the skin temperature fell to about 35xc2x0 C. and held there for at least the next two hours.
Table 3 represents laser Doppler data collected from the tissue during the experiments and correlates to blood flow through the local area being treated with heat. The baseline flow is approximately 80 ml/100 g/min and rises to about 200 ml/100 g/min at its peak, half way through the heating period. The flow xe2x80x9cnormalizesxe2x80x9d back to baseline during the last half of the heating period and remains at about baseline for the remainder of the measuring period.
The change in PsqO2 is followed in Table 4. The baseline PsqO2 is about 75 when heating begins and rises steadily to about 130 by the end of the heating period. The PsqO2 remains at this level for the remainder of the measuring period despite the lack of heating for the last 180 minutes. The added benefit of increased PsqO2 by heating continues well into the period of time after active heating has ceased. Wounds will continue to benefit from the effects of heating for substantial periods of time after the heating is turned off. The consequences of this study with the present invention is that the heating need not be constant, but deliverable over a heater therapy cycle or cycles that may nor may not be part of a larger therapeutic sequence.
Out initial human clinical data shows that the beneficial effects of heating on blood flow and PsqO2 last at least one hour longer than the actual duration of heat application. Further, we have noted that cycled heating seems to be more effective for wound healing than continuous heating. Therefore, the data recommends cycling the heater in a therapy cycle (e.g. 1 hour xe2x80x9conxe2x80x9d and 1 hour xe2x80x9coffxe2x80x9d) for a total heating time of 2-8 hours per day as a therapeutic sequence.
None of the 29 patients with compromised circulation treated to date have shown any indication of skin damage due to 38xc2x0 C. heat. Furthermore, none of these wounds have exceeded 35xc2x0 C. tissue temperature, with an average wound temperature of 32-33xc2x0 C. The present invention raises the wound temperature toward normothermia, but even on a poorly perfused leg, the tissue does not reach normothermia.
Accordingly, the invention concerns an apparatus for applying heat to tissue from a level that does not contact the tissue in order to elevate the temperature of the tissue being treated toward normothermia. This objective is accomplished by means of an apparatus having a ring that surrounds, but does not contact the tissue to be treated. This ring has a layer out of contact with the tissue to be treated at which a heater is located. The apparatus may be attached to the skin of a person so that the ring and the layer form a treatment volume which does not contact the tissue to be treated. The heater, supported at the layer, is held near the tissue to be treated, out of contact with the tissue. The heater may be an active heater, or an inactive, reflective heater. If active, the apparatus may include a programmable active heater controller. One form of an active heater is an electrical resistant filament which may have various geometric shapes in order to provide versatility in application of heat to the tissue to be treated.